There are a number of different circumstances in which it is desirable to perform analysis to identify compounds in a sample. Such samples may be taken directly from the environment or they may be provided by front end specialized devices to separate or prepare compounds before analysis. There exists, a demand for low cost, compact, low-power, accurate, easy to use, and reliable devices capable of detecting compounds in a sample.
One class of known analyzers are mass spectrometers (MS). Mass spectrometers are generally recognized as being the most accurate type of analyzers for compound identification. However, mass spectrometers are quite expensive, easily exceeding a cost of $100,000 or more and are physically large enough to become difficult to deploy everywhere the public might be exposed to dangerous chemicals. Mass spectrometers also suffer from other shortcomings such as the need to operate at relatively low pressures, resulting in complex support systems. They also need a highly trained operator to tend to and interpret the results. Accordingly, mass spectrometers are generally difficult to use outside of laboratories.
A class of chemical analysis instruments more suitable for field operation is known as Field Asymmetric Ion Mobility Spectrometers (FAIMS) or Differential Mobility Spectrometers (DMS), and also known as Radio Frequency Ion Mobility Spectrometers (RFIMS) among other names. Hereinafter, FAIMS, DMS, and RFIMS, are referred to collectively as DMS. This type of spectrometer subjects an ionized fluid (e.g., gas, liquid or vapor) sample to a varying asymmetric electric field and filters ions based on change to their ion mobility.
The sample flows through a filter field which allows selected ion species to pass through, according to a compensation voltage (Vcomp) applied to filter electrodes, and specifically those ions that exhibit particular mobility responses to the filter field. An ion detector then collects ion intensity/abundance data for the detected ions. The intensity data exhibits attributes, such as “peaks” at particular compensation voltages.
A typical DMS device includes a pair of electrodes in a drift tube. An asymmetric RF field is applied to the electrodes across the ion flow path. The asymmetric RF field, as shown in FIG. 1, alternates between a high or maximum field strength and a low field strength. The field varies over a particular time period (T), at a given frequency (f) and duty cycle (d). The field strength E varies with an applied RF field voltage (Vrf) and the size of the gap between the electrodes. Ions pass through the gap between the electrodes when their net transverse displacement per period of the asymmetric field is zero. In contrast, ions that undergo a net displacement eventually undergo collisional neutralization on one of the electrodes. In a given RF field, a displaced ion can be restored to the center of the gap (i.e. compensated, with no net displacement for that ion) by superimposing a low strength direct current (dc) electric field (e.g., by applying Vcomp across the filter electrodes) on the RF. Ions with differing displacement (owing to characteristic dependence of mobility in the particular field) pass through the gap at differing characteristic compensation voltages. By applying a substantially constant Vcomp, the system can be made to function as a continuous ion filter. Alternatively, scanning Vcomp obtains a spectral measurement for a sample. A recorded image of the spectral scan of the sample is sometimes referred to as a “mobility scan” or as an “ionogram.”
Examples of mobility scans based on the output from a DMS device are shown in FIGS. 2A and 2B. The compounds for which scans are depicted are acetone and an isomer of xylene (ortho-xylene). The scan of FIG. 2A resulted from a single compound, acetone, being independently applied to the DMS analyzer. The illustrated plot is typical of the observed response of the DMS device, with an intensity of detected ions dependent on Vcomp. For example, the acetone sample exhibits a peak intensity response at a Vcomp of approximately −2 Vdc.
FIG. 2B illustrates the results when analyzing a mixture of acetone and o-xylene. The combined response shows two peaks in approximately the same region as for the independent case. The compounds in the mixture can be detected by comparing the response against a library, for example, of stored known responses for independently analyzed compounds, or libraries of mixtures. Thus, the scans for independently analyzed compounds, such as the scan of FIG. 2A for acetone, can be stored in a computer system, and when compound responses such as that in FIG. 2B are observed, the relative locations of the peaks can be compared against the stored responses in the library to determine the identity of the compound.
A specific RF field voltage and compensation field voltage Vcomp permits only ion species having a particular ion mobility characteristic to pass through the filter to the detector. By noting the RF level and compensation voltage and the corresponding detected signal, various ion species can be identified, as well as their relative concentrations as seen in the peak characteristics, e.g., intensity, current, and voltage.
Consider a plot of ion mobility dependence on Vrf, as shown in FIG. 3. This figure shows ion intensity/abundancy versus RF field strength for three examples of ions, with field dependent mobility (expressed as the coefficient of high field mobility, a) shown for species at greater, equal to and less than zero. The velocity of an ion can be measured in an electric field (E) low enough so that velocity (v) is proportional to the electrical field as v=KE, through a coefficient (K) called the coefficient of mobility. K can be shown to be related to the ion species and gas molecular interaction properties. This coefficient of mobility is considered to be a unique parameter that enables the identification of different ion species and is determined by, ion properties such as charge, size, and mass as well as the collision frequency and energy obtained by ions between collisions.
When the ratio of E/N, where N is gas density, is small, K is constant in value, but at increasing E/N values, the coefficient of mobility begins to vary. The effect of the electric field can be expressed approximately as K(E)=K(0)[1+α(E)], where K(0) is a low voltage coefficient of mobility, and α is a specific parameter showing the electric field dependence of mobility for a specific ion.
Thus, as shown in FIG. 3, at relatively low electric field strengths, for example, of less than approximately 10,000 V/cm, multiple ions may have the same mobility. However, as the electric field strengths increase, the different species diverge in their response such that their mobility varies as a function of the applied electric field. This shows that ion mobility is independent of applied RF field voltage at relatively low RF field strengths, but is field-dependent at higher RF field strengths.
The ions passing through the filter are detected downstream. The detection signal intensity can be plotted as a characteristic detection peak for a given RF field voltage and compensation field voltage Vcomp. Peak intensity, location, and shape are typically used for species identification.
However, the peaks in a typical DMS spectra can be broad in width. Therefore, compounds exhibiting intensity peaks at similar compensation voltages may be difficult to separate from each another. Consequently, there may be particular conditions under which two different chemicals generate indistinguishable scans for a particular Vcomp and a particular RF field voltage, or for other combinations of filter field/flow channel parameters. In such a case, it is may not be possible to differentiate between the two different compounds. In other cases, two or more chemical species may have the same or almost the same ion mobility characteristic for a particular set of field/flow channel parameters. This is most likely to happen in the low electric field regime, where typical time-of-flight ion mobility spectrometer systems operate. Therefore, if two or more chemical species have the same or almost the same mobility characteristic, then their spectroscopic peaks will overlap, and identification and quantification of individual species will be difficult or impossible.
FIG. 4 is a graph of Vcomp versus Vrf that highlights the above described prior art limitations. More particularly, FIG. 4 depicts a graph of Vcomp versus Vrf for four compounds: lutidine; cyclohexane; benzene; and dimethyl-methyl-phosphonate (DMMP). Each curve shows the location of detected ion intensity peaks, such as those circled at 100, at the various (Vrf, Vcomp) locations, which in total provide the peak characteristics for each particular compound. As shown, there is a region 100 in which the intensity peaks and mobility curves for DMMP and cyclohexane overlap with each other. As can be seen, operating in a Vrf region of from approximately 2,500 Vpeak to approximately 2,650 Vpeak, at a Vcomp of about −6 Vdc to about −8 Vdc, one would find it virtually impossible to discriminate between the two compounds based on a single Vcomp scan at a single Vrf. Specifically, in a conventional spectral scan approach that plots intensity/abundance versus Vcomp over a range of Vcomp for a single Vrf plots the overlapping peaks as a single peak.
Recently, combined DMS and IMS analyzers have been used to more selectively discriminate between ion species that have exhibited overlapping spectra. However, the sensitivity of these DMS/IMS systems has been degraded by the interaction of neutrals or other particles with ions during the detection process. Thus, there is a need for an improved configurations and methods of operating ion mobility filters in combinations to provide more sensitive ion species detection.